Loose deposits of sands or silts which exist below the ground-water table are a hazard and require compaction or "densification" before structures are placed thereupon. If this were not done then subsequent settling of such soils (sands and/or silts), or failure of the soils due to liquefaction, beneath the structure would damage the structure and/or endanger the safety of persons or objects within or near the structure.
Where loose saturated sand exists at depth, (typically up to about 50 feet below ground level), a process termed "vibroflotation" is the currently accepted Civil Engineering technique for compacting or "densifying" the sand. There is an established body of literature which deals with vibroflotation, for example Loose Sands--Their Compaction By Vibroflotation, Elio D'Appolonia, 1969 A.S.T.M., pp. 138-162. As explained by D'Appolonia at page 139-140, "The process, as the name implies, employs mechanical vibrations and simultaneous saturation with water to move, shake, and `float` the sand particles into a dense state." D'Appolonia further explains in the text bridging pages 140-141 that water jets aid penetration of the vibroflot device into the sand. Once the vibroflot device has reached the desired depth, additional water is injected to aid in the compaction process. Thus, in practice, densification devices are relatively simple apparatus consisting of an extended conduit which is forced into the ground and then vibrated while water is added to the sand.
In his U.S. Pat. No. 4,664,557 entitled "Method and Apparatus for Constructing an Underwater Fill", Hodge introduced the idea of pumping water out of a submerged sand pile, while it is being constructed, to improve the engineering properties of the resulting sand mass. Pumping water out of the interior of the sand mass induces water to flow in through the outside faces of the accumulating sand pile. This circulation of water during the building process creates seepage forces which support the side slopes of the pile as it is formed, consequently enabling the formation of steeper outside slopes. Increased shear stresses created in the neighbourhood of steeper slopes are believed to promote a denser sand packing.
U.S. Pat. No. 4,699,546 Massarsch discloses a method of densifying a sand through application of a compacting force within the sand. Massarsch provides a perforated pipe which may function to drain water away from the region being compacted, but does not suggest the creation of seepage forces by actively withdrawing water through the region being densified.
The prior art discloses pumping water from a sump during vibratory compaction of a pre-existing soil. See for example published Japanese patent application Serial No. 56-4172 of Yoshihiko Kihara entitled "Vibrating Dehydration Promoting System Ground Consolidating Method and Device Thereof". Kihara provides a perforated pipe which is inserted into the ground and vibrated. Water flows into the pipe, through the perforations, and accumulates in the bottom interior region of the pipe. A pump lowered to the bottom of the pipe is used to discharge water which accumulates there to the surface through a drain hose.
Hodge in his South African patent No. 88/8485 granted 26 Jul. 1989 entitled "Method for Densification of Particulate Masses" (see also European Patent Application Publication No. 318,172 dated 31 May 1989; or, Japanese Patent Publication No. 244013/89 dated 28 Sep. 1989) teaches the benefits of actively withdrawing water from the region close to the application of vibratory densification energy. In that apparatus the drainage device is situated about five feet above the centre of application of the vibratory force. Kihara, like Massarsch or other vibroflotation processes which permit drainage of the ground locate the source of the vibration at the top of the apparatus, far away from the site of the drainage element.
Whereas sands are amenable to treatment by vibratory methods, silts are conventionally treated by the application of sustained static loading, for example temporarily placing a thick layer of sand on the ground surface over the area in question, and then, several months later, removing that sand again. The areal extent of the required "pre-load" increases as the depth to the deleterious soil increases, a fact which affects adjacent properties in many cases.
As saturated soil is made more dense, the spaces between individual grains of soil decrease in volume. The water which occupies those spaces becomes pressurized as the spacing between grains decreases, and this water tends to travel away from the region being densified, to regions of lesser pressure. This surplus water, which surrounds prior art devices, absorbs densification energy wastefully and buffers the effects of the compactive effort on the grains. The presence of entrapped water also delays the readjustment of grains between grains, and this delay means energy is being transmitted during that interval to no effect. Until the surplus water has exited the region, adjacent particles cannot come together in the more compact arrangement which produces a denser soil. The natural rate at which water can vacate the region being densified is of the order of hours in sand and of the order of weeks in silt. This time lag is one of the reasons that current devices are unable to densify silt.
The inventor believes that what is necessary in order to produce denser packing in both sands and silts is to cause particles to translate sub-horizontally relative to particles lying directly above and beneath. This movement must be accomplished to whatever extent possible, in the absence of excess "pore" water. The term "subhorizontal" is intended to imply a movement which is predominantly horizontal, but with a small vertically downward component (about two to five degrees). This allows particles to move over adjacent grains and to be then pushed, rather than fortuitously fall, into the void spaces between individual grains. The most direct, effective, and consequently, efficient method of accomplishing that objective is to move grains in a sub-horizontal radial direction in the region around the device, while providing for drainage of all surplus water. By restricting the energy output to the specific task of causing translation of soil in the preferred direction simultaneously in all radial directions, the energy is most efficiently utilized. The simultaneous withdrawal of water from the soil complements the translational movements because those aligned hydraulic drag forces exerted by the flowing water on the soil particles pull (attract) the soil particles towards the device in a radial direction.
By contrast the prior art devices are rigid bodied steel pipes without expandable sections; essentially they shake a pipe in the ground. Where they incorporate a drainage element, that drainage element will either fail to prevent silt particles from entering the body of the device, or it will be rendered incapable of admitting water because of smearing of the porous barrier. Also, where drainage elements are incorporated into the design the drain is sufficiently far removed from the focus of the vibratory energy, and the attendant localized pore water pressure generation, that energy and time are both wasted.